Recently, as networks are widely used, conventional printed documents are increasingly replaced with documents displayed on a flexible display device, which is widespread throughout the market of electronic publishing industry including the markets of books, magazines, and the like.
Users may view such documents through a cathode-ray tube (CRT) or a liquid crystal display (LCD) of a computer to read information in the documents. However, the CRT as or the LCD of a computer is a display device of a light emitting type, it is difficult for users to read information for a long time due to eye fatigue felt when using the display device for an extended period of time, and such documents may only be read in a place where a computer, an LCD, or the like is installed.
Although a portable display device may be used as an alternative with a widespread distribution of notebook type computers, the portable display device also uses a light emitting display with a backlight, causing a problem in that power consumption limits reading for an extended period of time.
Reflective type liquid crystal display devices that can be driven with low power consumption have been developed and released. However, in the white mode, reflective liquid crystal display devices have low reflectance, which leads to remarkably low visibility, and causes fatigue more easily compared to printed paper materials, such that it is unable to solve the problems described above, and may not be used for reading for an extended period of time.
In order to solve the above problems, the so-called electronic paper is being developed recently.
The electronic paper is a core element for implementing a flexible display, which makes a motion when an electromagnetic field is applied to a conductive material. That is, after distributing charged particles between thin film flexible substrates, data are expressed by changing arrangement of directions of the charged particles based on changes of polarities of the electromagnetic field.
In this case, if the arrangement of directions of the charged particles occurs at any polarity, an image is maintained as it is since positions of the particles are unchanged due to memory effect even though voltage is removed, thereby obtaining an effect of printing the image on a paper with ink. Accordingly, since the electronic papers does not have self-emitting light, visual fatigue is remarkably lowered, and thus it is possible to view documents with comfort like reading a real book. In addition, since flexibility and portability are secured by using a flexible substrate, the electronic paper is highly expected as a future flat panel display technique.
In addition, if an image is embodied once, it is maintained for an extended period of time as described above until the substrate is re-set, such that power consumption may be very low, and the electronic paper may be conveniently used as a portable display device.
As manufacturing cost of the electronic paper is extremely low compared to conventional flat panel displays, and background illumination or continuous recharge of battery is not required, the electronic paper may be driven with an extremely low energy, and thus they are remarkably superior in terms of energy efficiency.
With the advantages described above, the electronic paper is applicable to a variety of fields, including electronic books and newspapers having paper-like surfaces and mobile illustrations, reusable paper displays for cellular phones, disposable TV screens, electronic wallpapers, and the like, so that a vast potential market may be expected.
In a flexible display device such as the electronic paper described above, an upper electrode having charged particles of an image is required to be combined with a lower electrode to which voltage is applied, in which the flexible display device is manufactured by inserting an adhesive film between the two electrodes.
However, there may be a loss in driving voltage or inconsistent driving may occur due to the adhesive film, and a charged particle layer of image may be damaged due to a heating and pressing process in the process of manufacturing the adhesive film. Therefore, there is a need to manufacture an adhesive film having a low voltage loss, which may be accomplished by a uniform adhesive means of the adhesive film.
For example, Korean Laid-open Patent Publication No. 2006-0032111 discloses a method of bonding barrier ribs and a substrate in a thermal bonding method after transcribing an adhesive means on the top and bottom of a transparent electrode using adhesive means that have different transition temperatures.
However, since instability of the barrier ribs, a pigment, or a toner increases while they pass through the thermal bonding process twice, it is difficult to practically utilize the method.
Korean Laid-open Patent Publication No. 2006-0067006 discloses a bonding method using an ultraviolet lamp, in which an ultraviolet curing adhesive is used as an adhesive means. Further, Korean Patent No. 10-1030936 discloses an electrophoretic display, in which a fluid composition and a liquid crystal composition are used to form a sealing layer and an adhesive layer, which is made from a radiation-curable composition and attached to a second electrode layer (e.g., an electrode such as a thin film transistor (TFT)). However, filled-in pigment or toner are mixed with ultraviolet curable monomers, such that the pigment or toner may be fixed to the adhesive layer after a curing process, or an ultraviolet curing process may not occur. In addition, most of the pigment used in this method is unstable with ultraviolet rays, requiring a function of blocking ultraviolet rays from outside, making this bonding method unrealistic.
An attempt has been made to improve the problems of conventional thick adhesive films, as disclosed in Korean Laid-open Patent Publication No. 2007-0041197, in which an EVA adhesive is implemented as a thin film using a vacuum evaporation method. However, it is difficult to apply a desired amount of EVA adhesive, which is an organic material, at a desired position using the vacuum evaporation method.
Korean Laid-open Patent Publication No. 2011-0032357 discloses a technique for bonding barrier ribs and a substrate at a desired position using a tape configured with a flexible film placed in the middle and two adhesive layers attached on both sides of the flexible film. However, although the barrier ribs and the substrate may be attached at a desired position, a quality resolution on a display material may not be achieved using a double-sided tape.
In addition, due to strong adhesiveness, the adhesive layer applied to an electronic paper among the flexible displays described above does not allow a re-work on each material if the pigment or toner is defective at the time of attachment or if there is a problem in the upper or lower electrode. Accordingly, a loss occurred during manufacturing of the adhesive is enormous since relatively expensive thin film transistors (TFTs) cannot be used. Furthermore, the Korean Patent rarely discloses properties of an adhesive layer interposed between an upper or lower electrode, thereby failing to propose a method for overcoming a loss in the driving voltage.
Therefore, in an effort to solve the conventional problems, inventors of the present invention have produced the present invention, in which a lower electrode, to which voltage is applied, and an image upper electrode coated with charged particles, of which colors are changed depending on the applied voltage, are attached to the adhesive film, and dielectric characteristics are controlled by controlling a dielectric constant of an adhesive film, such that a resistance value in a thickness direction may be controlled, thereby manufacturing an adhesive film without a loss in the driving voltage.